Synthesis, characterization and properties of 1,2,4-triazolo[3,4-b][1,3,4] thiadiazole derivatives and their europium complexes

Article abstract of DOI:10.1016/j.molstruc.2014.06.026

Eight novel 1,2,4-triazolo[3,4-b][1,3,4]thiadiazole derivatives and their corresponding Eu(III) complexes were synthesized and characterized. From the spectral studies it has been concluded that the title Eu(III) complexes display six coordination. The investigation of the luminescence properties of the title complexes showed that the ligands favor energy transfers to the emitting energy level of europium ions. The title Eu(III) complexes exhibited characteristic emissions of europium ions, and possessed strong luminescence intensities, good fluorescence quantum yields, and the highest fluorescence quantum yield is up to 0.522. Furthermore, the luminescence intensity of the complex with chlorine-substituted group is the strongest than that of other complexes. The exploration of the electrochemical properties of the title complexes shows that the introduction of electron-donating groups can increase the HOMO energy levels, LUMO energy levels and the oxidation potential of the complexes, however, the result of introduction of electron-withdrawing groups was just opposite.

Full text of DOI:10.1016/j.molstruc.2014.06.026

Journal of Molecular Structure 1074 (2014) 487–495
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, characterization and properties of 1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole derivatives and their europium complexes
Dong Yan a, Yu Xiang a, Kangyun Li a, Yanwen Chen b, Zehui Yang c, Dongcai Guo a,
⇑
a School of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
b Hunan Labour Protection Institute of Nonferrous Metals, Changsha 410014, China
c School of Chemical Engineering, Ningbo University of Technology, Ningbo 315016, China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
Eight novel europium complexes have
been prepared and characterized.
All the target europium complexes
emit characteristic fluorescence of
europium ions.
ꢀ
ꢀ
The relationship between the
structure of ligands and luminescence
intensity of the europium complexes
have been discussed.
The electrochemical properties of the
title complexes have been estimated.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 April 2014
Received in revised form 27 May 2014
Accepted 9 June 2014
Available online 19 June 2014
Eight novel 1,2,4-triazolo[3,4-b][1,3,4]thiadiazole derivatives and their corresponding Eu(III) complexes
were synthesized and characterized. From the spectral studies it has been concluded that the title Eu(III)
complexes display six coordination. The investigation of the luminescence properties of the title com-
plexes showed that the ligands favor energy transfers to the emitting energy level of europium ions.
The title Eu(III) complexes exhibited characteristic emissions of europium ions, and possessed strong
luminescence intensities, good fluorescence quantum yields, and the highest fluorescence quantum yield
is up to 0.522. Furthermore, the luminescence intensity of the complex with chlorine-substituted group is
the strongest than that of other complexes. The exploration of the electrochemical properties of the title
complexes shows that the introduction of electron-donating groups can increase the HOMO energy lev-
els, LUMO energy levels and the oxidation potential of the complexes, however, the result of introduction
of electron-withdrawing groups was just opposite.
Keywords:
Triazolo
Complexes
Rare earth
Synthesis
Electrochemical properties
Luminescence
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
Abbreviations: NMR, nuclear magnetic resonance; UV–Vis, ultraviolet visible;
EDTA, ethylenediaminetetraacetic acid; HOMO, highest occupied molecular orbital;
LUMO, lowest unoccupied molecular orbital.
Rare earth complexes have quickly become the focus of
research in academia on the basis of their specific physical and
chemical properties. Rare earth organic complexes often show
good optical properties, mainly due to their large stockes shifts,
⇑
Corresponding author. Tel./fax: +86 0731 88821449.
E-mail address: dcguo2001@hnu.edu.cn (D. Guo).
http://dx.doi.org/10.1016/j.molstruc.2014.06.026
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

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D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
narrow emission bandwidths and long luminescence lifetimes
[1,2], and they have been widely used in biomedical and functional
materials [3], such as lasers materials [4], analytical and structure
probe [5,6], and light emitting diodes [7,8]. The efficiency of ligand
absorption in the UV region, ligand-to-metal energy transfer and
rare earth luminescence play an important role in determining
luminescence intensity [9]. Therefore, it is significant to design
and synthesis the ligand, whose triplet energy level matches better
to the emitting level of the rare earth ions. The 1,2,4-triazolo[3,4-
b][1,3,4]thiadiazole ring systems have been extensively studied
due to their properties with respect to versatile biological activities
including antibacterial, antifungal, anticancer, antitubercular and
analgesic activities and other potential applications [10,11]. How-
ever, 1,2,4-triazolo-[3,4-b][1,3,4]thiadiazoles have been seldom
reported in literature with their corresponding rare earth ions
complexes due to their poor solubility, not to mention the lumines-
cence properties of their rare earth complexes. On the other hand,
the triazole–thiadiazole compounds possess good conjugated
plane, rigid structure and various coordination sites, are theoreti-
cally suitably used as the organic ligand of the rare earth lumines-
cent complexes.
In this paper, we tried to design and synthesize a series of novel
triazolo-thiadiazole systems and their Eu(III) complexes, investi-
gated the luminescent and electrochemical properties of the com-
plexes and explored the relationship between the structure of the
ligand and the performance of complexes. This work might provide
a reference for the development of the luminescent materials. The
synthetic route of the 2,6-bis(4-(4-substituted phenoxymethyl)-
1,2,4-triazolo[3,4-b][1,3,4]thiadiazole)-pyridine derivatives is
depicted in Scheme 1.
Methods
The melting points were determined on a TECH XT-4 binocular
microscopic melting point apparatus (made in Beijing, China) and
uncorrected. The europium ion was determined by ethylenedi-
aminetetraacetic acid (EDTA) titration using xylenol orange as an
indicator. The carbon, hydrogen, and nitrogen analyses were car-
ried out using an Elementar Vario EL-III elemental analyzer, 1H
NMR spectra were measured on a Varian-400 NMR (400 MHz)
spectrometer (both from Agilent Technologies Co., Ltd., Beijing,
China), spectra were taken in CDCl3 or DMSO-d6 solution using tet-
ramethylsilane (TMS) as an internal reference. IR spectra in the
4000–400 cmꢁ1 region were measured using KBr pellets on a
PerkinElmer spectrometer (PerkinElmer Chengdu, Chengdu,
China). UV spectra were recorded on
a LabTech UV-2100
(Columbia MO, USA) spectrophotometer using DMSO to dissolve
the samples. Fluorescence measurements were made with a
Hitachi F-2700 fluorescence spectrophotometer at room tempera-
ture. Differential scanning calorimetry (DSC) and thermogravimet-
ric analysis (TGA) was performed by NETZSCH STA 409PC
simultaneous thermal analyzer with a heating rate of 10 °C/min
under a nitrogen atmosphere. Cyclic voltammetry (CV) measure-
ments were performed using a CHI 660d electrochemical worksta-
tion under nitrogen. A three-electrode cell is equipped which
consist of a glass carbon working electrode, Pt wire as auxillary
electrode, standard calomel electrode as reference electrode and
using ferrocene (EOX = 0.3 eV) as an external standard, sodium
nitrite solution (0.1 M) as electrolyte, DMSO as solvent. The cyclic
voltammogram was recorded at a scan speed of 100 mV sꢁ1 at a
sensitivity of 1 mA.
General procedure for the synthesis of the intermediates
Experimental
Synthesis of pyridine-2,6-dihydrazide (1)
Materials
Pyridine-2,6-dimethyl ester (53 mmol, 10.34 g) and anhydrous
ethanol (200 mL) were added into a 500 mL three-necked flask,
the mixture was heated to 85 °C. Next, hydrazine (30 mL, 85%) was
added to the mixture and refluxed for 6 h. Then the reaction mixture
was distilled under reduced pressure in order to remove most of the
solvent, and diluted with some distilled water, and then a large
Pyridine-2,6-dicarboxylate and Eu(NO3)3 were prepared accord-
ing to literature methods [12,13]. Chloroacetic acid and ethyl ace-
tate were of CP grade, phenol derivatives and other reagents were
of AR grade, and used without further purification.
Scheme 1. Synthetic route of the ligands L1–8
.

D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
489
amount of solid white precipitate was obtained. This precipitated
solid compound was filtered off, washed three times with water,
and recrystallized from water, filtered again and dried under vac-
uum and finally formed the white acicular crystals, yield: 91%.
p-Fluorophenoxyacetic acid (4e). White powder, yield 52%. 1H NMR
(400 MHz, CDCl3) d/ppm: 4.67 (s, 2H, CH2), 6.87–6.90 (m, 2H, ArH),
6.99–7.03 (m, 2H, ArH); MS (EI) m/z (%): 171 (M+1, 9), 170 (M,
100), 125 (79), 112 (41), 95 (68), 83 (30), 75 (17).
Synthesis of 5,50-(2,6-pyridyl)-bis-4-amino-5-thiol-1,2,4-triazole (2)
The procedures were carried out and revised according to Ref.
[14]. Pyridine-2,6-dihydrazide (5 mmol, 0.98 g) was mixed with a
solution of potassium hydroxide (20 mmol, 1.12 g) in anhydrous
ethanol (30 mL) under stirring. Then, a solution of carbon disulfide
(20 mmol, 1.52 g) in ethanol (10 mL) was added dropwise. The
reaction mixture was stirred for 14 h at room temperature. After
that ether (30 mL) was added into the mixture and kept stirring
for 0.5 h. The solid product of potassium salt was collected by fil-
tration and vacuum drying. It was used for the next reaction
directly without further purification.
The above product was poured into the mixture solution of
hydrazine hydrate (2 mL, 85%) and deionized water (2 mL), then
the resulting mixture was refluxed for 6 h at 110 °C, cooled with
cold water and its pH value was adjusted to 6 with diluted hydro-
chloric acid, then a large precipitate was separated. The solid was
allowed to stand overnight, filtrated and poured into a solution
of ethanol and water solution (1:1, 30 mL), heated to boiling under
stirring. The title product was collected by filtration and vacuum
drying for 24 h with a yield 20%.
p-Chlorophenoxyacetic acid (4f). White crystals, yield 85%. 1H NMR
(400 MHz, CDCl3) d/ppm: 4.67 (s, 2H, CH2), 6.87 (d, 2H, J = 8.8 Hz,
ArH), 7.27 (d, 2H, J = 8.4 Hz, ArH); MS (EI) m/z (%): 188 (M+2, 32),
186 (M, 100), 141 (79), 128 (56), 111 (59), 99 (32), 75 (33).
p-Bromophenoxyacetic acid (4g). White powder, yield 78%. 1H NMR
(400 MHz, CDCl3) d/ppm: 4.76 (s, 2H, CH2), 6.82 (d, 2H, J = 9.2 Hz,
ArH), 7.42 (d, 2H, J = 9.2 Hz, ArH); MS (EI) m/z (%): 232 (M+1, 97),
230 (Mꢁ1, 100), 187 (47), 185 (49), 174 (34), 172 (37), 157 (40),
155 (35), 143 (17), 76 (20).
p-Iodophenoxyacetic acid (4h). White powder, yield 77%. 1H NMR
(400 MHz, CDCl3) d/ppm: 4.67 (s, 2H, CH2), 6.71 (d, 2H, J = 9.2 Hz,
ArH), 7.60 (d, 2H, J = 8.4 Hz, ArH); MS (EI) m/z (%): 279 (M+1, 9),
278 (M, 100), 233 (23), 220 (21), 203 (15), 191 (7), 106 (6), 92
(6), 76 (11).
Synthesis of 1,2,4-triazolo[3,4-b][1,3,4]thiadiazole derivatives (L1–8
)
Synthetic methods of compounds L1–8 were similar with each
other, we took the synthetic method of the compound L1 for exam-
ple. A mixture of obtained compound 2 (2 mmol, 0.61 g), phenoxy-
acetic acid (6 mmol, 0.91 g) and phosphorus oxychloride (15 mL)
were added to a 25 mL single-necked flask, then the reaction mix-
ture was heated to 106 °C and refluxed for 7 h. The solvent was
removed under vacuum distillation and cooled at room tempera-
ture. Then the residue was poured into ice-water mixture
(400 mL) under stirring. The pH value of the mixture was adjusted
to 7–8 with diluted NaOH solution. Then the precipitated solid was
separated by suction filtration, and washed with water to be neu-
tral. The crude product was dried and added to anhydrous ethanol
(100 mL), heated to boiling and separated by suction filtration,
purified by washing thoroughly with anhydrous ethanol and dried
in vacuum to give the title product L1.
Synthesis of phenoxyacetic acid derivatives (4a–h)
Compounds 4a–h were prepared by similar procedures. In a
typical synthesis of 4a, monochloroacetic acid (0.05 mol, 4.72 g)
was dissolved in deionized water (20 mL). The solution was cooled
in an ice bath and NaOH was added with stirring until the pH value
was adjusted to 9–10, then a solution of sodium chloroacetate was
obtained. A mixture of NaOH (0.04 mol, 1.60 g), deionized water
(20 mL) and ethanol (20 mL) were poured into a 150 mL three-
necked flask, then phenol (0.04 mol, 3.76 g) was slowly added
under stirring. Twenty minutes later, the above sodium chloroace-
tate was added dropwise. The reaction solution was heated to
105 °C and refluxed for 5 h. After cooling down, the pH value of
the mixture was acidified to 1–2 with diluted hydrochloric acid.
The precipitate was collected by filtration and washed with diluted
hydrochloric acid many times. Recrystallized and dried under a
vacuum, resulting in a white solid product of the phenoxyacetic
acid (4a).
3,30-(2,6-Pyridyl)-bis-6-phenoxymethyl-1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole (L1). Brown solid, yield 81%. m.p. 192–194 °C; 1
H
NMR (400 MHz, CDCl3) d/ppm: 5.45 (s, 4H, CH2), 6.98–7.00 (d, d;
4H, J1 = 8.8 Hz, J2 = 0.8 Hz, ArH), 7.06–7.10 (m, 2H, ArH), 7.31–
7.35 (m, 4H, ArH), 8.11–8.15 (t, 1H, J = 7.8 Hz, pyridine proton),
8.49 (d, 2H, J = 8 Hz, pyridine protons); IR (KBr) v/cmꢁ1: 3138,
1598, 1573, 1540, 1454, 1401, 1246, 1080, 690; MS (ESI) m/z (%):
1079 (2Mꢁ1, 47), 563 (M+23, 15), 562 (M+22, 53), 540 (M, 51);
Anal. Calcd. for C25H17N9O2S2: C, 55.65; H, 3.18; N, 23.36; S,
11.88. Found: C, 55.25; H, 3.62; N, 23.75; S, 11.46.
Phenoxyacetic acid (4a). White crystals, yield 67%. 1H NMR
(400 MHz, CDCl3) d/ppm: 4.70 (s, 2H, CH2), 6.94 (d, 2H, J = 8.0 Hz,
ArH), 7.02–7.05 (t, 1H, J = 7.2 Hz, ArH), 7.30–7.35 (m, 2H, ArH);
MS (ESI) m/z (%): 304 (2M, 13), 303 (2Mꢁ1, 100), 151 (Mꢁ1, 28).
p-Methylphenoxyacetic acid (4b). White crystals, yield 70%. 1H NMR
(400 MHz, CDCl3) d/ppm: 2.30 (s, 3H, CH3), 4.66 (s, 2H, CH2), 6.83
(d, 2H, J = 8.8 Hz, ArH), 7.12 (d, 2H, J = 8.8 Hz, ArH); MS (EI) m/z
(%): 167 (M+1, 10), 166 (M, 100), 121 (60), 107 (50), 91 (64), 77
(26), 65 (17).
3,30-(2,6-Pyridyl)-bis-6-p-methylphenoxymethyl-1,2,4-triazolo[3,4-
b][1,3,4]thiadiazole (L2). Brown solid, yield 92%. m.p. 205–206 °C;
1H NMR (400 MHz, CDCl3) d/ppm: 2.31 (s, 6H, CH3), 5.41 (s, 4H,
CH2), 6.88 (d, 4H, J = 8.8 Hz, ArH), 7.06–7.15 (m, 4H, ArH), 8.10–
8.14 (t, 1H, J = 7.8 Hz, pyridine proton), 8.48 (d, 2H, J = 8.0 Hz, pyr-
idine protons); IR (KBr) v/cmꢁ1: 3122, 2860, 1590, 1574, 1542,
1458, 1401, 1241, 1083, 685; MS (ESI) m/z (%): 1157 (2M+21,
100), 1135 (2Mꢁ1, 40), 590 (M+22, 58), 568 (M, 47); Anal. Calcd.
for C27H21N9O2S2: C, 57.13; H, 3.73; N, 22.21; S, 11.30. Found: C,
57.35; H, 3.57; N, 22.15; S, 11.12.
p-Methoxyphenoxyacetic acid (4c). White crystals, yield 68%. 1H
NMR (400 MHz, CDCl3) d/ppm: 3.78 (s, 3H, CH3), 4.63 (s, 2H,
CH2), 6.84–6.86 (m, 2H, ArH), 6.87–6.90 (m, 2H, ArH); MS (EI)
m/z (%): 183 (M+1, 6), 182 (M, 64), 123 (100), 109 (16), 95 (26),
92 (9), 77 (8).
p-Nitrylphenoxyacetic acid (4d). White powder, yield 72%. 1H NMR
(400 MHz, DMSO-d6) d/ppm: 4.88 (s, 2H, CH2), 7.13–7.15 (t, 2H,
ArH), 8.20–8.22 (t, 2H, ArH); MS (EI) m/z (%): 198 (M+1, 9), 197
(M, 100), 181 (3), 167 (17), 152 (85), 139 (11), 122 (22), 109
(38), 92 (29), 76 (18).
3,30-(2,6-Pyridyl)-bis-6-p-methoxyphenoxymethyl-1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole (L3). Brown solid, yield 63%. m.p. 191–193 °C; 1
H
NMR (400 MHz, CDCl3) d/ppm: 3.77 (s, 6H, OCH3), 5.38 (s, 4H, CH2),
6.84 (d, 4H, 8.8 Hz, ArH), 6.92 (d, 4H, J = 9.2 Hz, ArH), 8.10–8.14 (t,
1H, J = 7.8 Hz, pyridine proton), 8.48 (d, 2H, J = 8.0 Hz, pyridine pro-

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D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
tons); IR (KBr) v/cmꢁ1: 3121, 1594, 1574, 1460, 1401, 1232, 1109,
1032, 688; MS (ESI) m/z (%): 623 (M+23, 38), 622 (M+22, 100);
Anal. Calcd. for C27H21N9O4S2: C, 54.08; H, 3.53; N, 21.02; S,
10.69. Found: C, 54.50; H, 3.58; N, 20.53; S, 10.67.
by suction filtration, purified by washing thoroughly with chloro-
form, and dried in a vacuum oven for 6 h.
Results and discussion
3,30-(2,6-Pyridyl)-bis-6-p-nitrylphenoxymethyl-1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole (L4). White solid, yield 61%. m.p. 309–310 °C; 1H
NMR (400 MHz, CDCl3) d/ppm: 5.47 (s, 4H, CH2), 7.21 (t, 4H,
ArH), 8.33 (t, 4H, ArH), 8.13–8.18 (t, 1H, J = 6.0 Hz, pyridine pro-
Properties of the title complexes
Analytical data for the newly synthesized complexes are listed
in Table 1. The results of the elemental analyses are in good agree-
ment with the theoretical values calculated. The conductance val-
ues are in the range of 151–202 (S cm2 molꢁ1), indicating that the
europium nitrate complexes conform to a kind of 1:2 electrolytes
[15]. The conductivity values in Table 1 suggest that one nitrate
is coordinated to the Eu ion. Ligands were easily dissolved in
DMSO, DMF, slightly soluble in chloroform, but hardly soluble in
water as well as acetone, methanol, ethanol, diethyl ether. All the
complexes are soluble in DMF, DMSO. The elemental contents (C,
H, N and Eu) of the Eu(III) complexes are quite close to those cal-
culated from molecular formula proposed, which indicates the cor-
rectness of molecular composition proposed.
ton), 8.48 (d, 2H, J = 6.0 Hz, pyridine protons); IR (KBr) v/cmꢁ1
:
3119, 1611, 1544, 1509, 1466, 1400, 1259, 1054, 687; MS (ESI)
m/z (%): 653 (M+23, 36), 652 (M+22, 100), 630 (M, 22); Anal. Calcd.
for C25H15N11O6S2: C, 47.69; H, 2.40; N, 24.47; S, 10.19. Found: C,
47.88; H, 2.41; N, 24.68; S, 10.14.
3,30-(2,6-Pyridyl)bis-6-p-fluorophenoxymethyl-1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole (L5). Gray solid, yield 92%. m.p. 228–230 °C; 1H
NMR (400 MHz, CDCl3) d/ppm: 5.40 (s, 4H, CH2), 6.93–6.98 (m,
4H, ArH), 7.00–7.05 (t, 4H, ArH), 8.11–8.15 (t, 1H, J = 8.0 Hz, pyri-
dine proton), 8.48 (d, 2H, J = 8.0 Hz, pyridine protons); IR (KBr)
v/cmꢁ1: 3118, 1593, 1574, 1542, 1464, 1402, 1251, 1098, 1081,
687; MS (ESI) m/z (%): 599 (M+23, 25), 598 (M+22, 100), 576 (M,
30); Anal. Calcd. for C25H15F2N9O2S2: C, 52.17; H, 2.63; N, 21.90;
S, 11.14. Found: C, 52.14; H, 2.53; N, 21.92; S, 11.11.
UV–Vis spectra
The UV–Visible absorption spectra data of the free ligands and
the corresponding Eu(III) complexes, which were recorded in
3,30-(2,6-Pyridyl)-bis-6-p-chlorophenoxymethyl-1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole (L6). White solid, yield 67%. m.p. 258–261 °C; 1H
NMR (400 MHz, CDCl3) d/ppm: 5.41 (s, 4H, CH2), 6.93 (d, 4H,
J = 8.8 Hz, ArH), 7.29 (d, 4H, J = 9.2 Hz, ArH), 8.11–8.15 (t, H,
J = 7.8 Hz, pyridine proton), 8.48 (d, 2H, J = 8.0 Hz, pyridine pro-
tons); IR (KBr) v/cmꢁ1: 3138, 1595, 1572, 1546, 1464, 1400,
1243, 1040, 709, 687; MS (ESI) m/z (%): 632 (M+24, 83), 630
(M+22, 100), 610 (M+2, 6), 608 (M, 16); Anal. Calcd. for C25H15Cl2-
N9O2S2: C, 49.35; H, 2.48; N, 20.72; S, 10.54. Found: C, 49.60; H,
2.49; N, 20.47; S, 10.60.
DMSO solution, and the molar absorptivities (e) are listed in
Table 2. Since the UV spectra of all the complexes exhibit the sim-
ilar features, only the UV spectra of [EuL2(NO3)](NO3)2ꢂH2O and
[EuL7(NO3)](NO3)2ꢂH2O as well as their corresponding ligands have
been selected to show in Figs. 1 and 2, respectively.
Figs. 1 and 2 display that the absorption peaks of L2 and L7
appear at 274 nm and 310 nm, 276 nm and 310 nm, respectively,
which are assigned to the
p ?
pꢃ transition of aromatic ring and
n ? pꢃ transition of conjugation between the lone pair of electrons
of p-orbital of N-atom in C@N [16], respectively. In comparison
with the bands of free ligands, red shift is observed in the bands
of complexes. Moreover, the band of
p ?
pꢃ transition is more evi-
3,30-(2,6-Pyridyl)-bis-6-p-bromidephenoxymethyl-1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole (L7). Gray solid, yield 83%. m.p. 244–246 °C; 1H
NMR (400 MHz, CDCl3) d/ppm: 5.40 (s, 4H, CH2), 6.88 (d, 4H,
J = 9.2 Hz, ArH), 7.43 (d, 4H, J = 9.2 Hz, ArH), 8.11–8.15 (t, 1H,
J = 7.8 Hz, pyridine proton), 8.48 (d, 2H, J = 8.0, pyridine protons);
IR (KBr) v/cmꢁ1: 3140, 1594, 1581, 1546, 1464, 1403, 1246, 1052,
680, 599; MS (ESI) m/z (%): 720 (M+23, 100), 718 (M+21, 49),
698 (M+1, 38), 696 (Mꢁ1, 37); Anal. Calcd. for C25H15Br2N9O2S2:
C, 43.06; H, 2.17; N, 18.08; S, 9.20. Found: C, 43.05; H, 2.14; N,
18.27; S, 9.34.
dent than that of n ? pꢃ transition. The UV spectral shapes of the
complexes are similar to that of the free ligand, which suggests
that the coordination of the Eu(III) ions does not have a significant
influence on the
p ?
pꢃ state energy, and also reveals that the
absorption of the complex is mainly attributed to the ligand. How-
ever, there is a little difference that the second strong absorption
spectra’s shapes of the complex shows a certain degree of change
compared with the corresponding free ligand, respectively,
because the electron density of complex is higher than that of
the ligands, which is due to the perturbation induced by the euro-
pium ions coordination. UV–Vis spectra show that the values of the
maximum absorption wavelength (kmax) for europium complexes
are higher than that of the ligands, with a slight change of the wave
number. On the other hand, the absorption intensity of the com-
plexes is increased.
3,30-(2,6-Pyridyl)-bis-6-p-iodophenoxymethyl-1,2,4-triazolo[3,4-b]
[1,3,4]thiadiazole (L8). Black solid, yield 88%. m.p. 155–156 °C; 1H
NMR (400 MHz, CDCl3) d/ppm: 5.40 (s, 4H, CH2), 6.76 (d, 4H,
J = 9.2, ArH), 7.62 (d, 4H, J = 9.2 Hz, ArH), 8.11–8.15 (t, 1H,
J = 8.0 Hz, pyridine proton), 8.48 (d, 2H, J = 8.4 Hz, pyridine
protons); IR (KBr) v/cmꢁ1: 3152, 1634, 1578, 1547, 1467, 1400,
1244, 1050, 680, 419; MS (ESI) m/z (%): 814 (M+23, 100), 792
(M+1, 14); Anal. Calcd. for C25H15I2N9O2S2: C, 37.94; H, 1.91; N,
15.93; S, 8.10. Found: C, 37.82; H, 1.93; N, 16.04; S, 8.18.
IR spectra
The IR spectral data of the ligands and their europium com-
plexes are listed in Table 3. IR spectra of all the Eu(III) complexes
displays the same manner, herein we just present the spectra of
[EuL2(NO3)](NO3)2ꢂH2O and [EuL5(NO3)](NO3)2ꢂH2O as well as their
ligands in Figs. 3 and 4, respectively.
Synthesis of the title europium complexes
Synthetic methods of the title complexes were similar, so we
took the synthetic method of the complex of compound L1 for
example. A mixture solution of methanol (1.25 mL) and europium
nitrate (1.25 mL, 0.2 mol Lꢁ1) was added dropwise to a solution of
compounds L1 (0.25 mmol) in chloroform (30 mL). The mixture
was stirred at 80 °C for 4 h. And then the precipitate was separated
According to the data in Table 3, the IR spectra of the uncoordi-
nated ligands shows that the bands centered at 1590–598 cmꢁ1 are
assigned to
region 1232–1259 cmꢁ1, 1032–1080 cmꢁ1 and 680–690 cmꢁ1 are
attributed to (C@NAN), (ArAOAC) and (CASAC), respectively.
m(C@N) of the pyridine ring, bands observed in the
m
m
m

D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
491
Table 1
Elemental analytical and molar conductance data of the complexes.
Complexes
Found (calculated) (%)
C
Km
H
N
Eu
(S cm2 molꢁ1
)
[EuL1(NO3)](NO3)2ꢂH2O
[EuL2(NO3)](NO3)2ꢂH2O
[EuL3(NO3)](NO3)2ꢂH2O
[EuL4(NO3)](NO3)2ꢂH2O
[EuL5(NO3)](NO3)2ꢂH2O
[EuL6(NO3)](NO3)2ꢂH2O
[EuL7(NO3)](NO3)2ꢂH2O
[EuL8(NO3)](NO3)2ꢂH2O
33.38(33.48)
35.01(35.06)
33.76(33.89)
30.41(30.43)
32.19(32.19)
31.14(31.12)
28.47(28.49)
26.05(26.15)
2.14(2.12)
2.31(2.49)
2.29(2.41)
1.65(1.72)
1.81(1.82)
1.78(1.76)
1.60(1.61)
1.49(1.48)
18.75(18.75)
18.16(18.18)
17.58(17.57)
19.85(19.88)
18.09(18.03)
30.32(20.33)
18.56(18.61)
14.55(14.65)
16.87(16.96)
16.27(16.45)
15.86(16.00)
15.43(15.42)
16.23(16.31)
15.74(15.77)
14.40(14.43)
13.27(13.25)
158
151
166
202
174
185
178
196
Table 2
1.2
0.8
0.4
0.0
-0.4
UV–Vis data of the complexes and ligands in DMSO solution (5 ꢂ 10ꢁ5 mol Lꢁ1).
Compounds
k1
e1
k2
e2 (1.0 ꢂ 104
(nm) (1.0 ꢂ 104 L molꢁ1 cmꢁ1
)
(nm) L molꢁ1 cmꢁ1
)
b
a
L1
275
2.48
1.70
1.87
1.54
1.96
1.84
1.62
1.16
1.86
1.14
1.59
1.58
1.77
1.56
1.92
1.47
310 2.32
311 1.67
310 1.60
313 1.29
304 2.09
305 1.97
311 1.76
313 1.32
311 1.75
313 0.86
312 1.53
312 1.65
310 1.72
313 1.60
310 1.85
311 1.37
[EuL1(NO3)](NO3)2ꢂH2O 277
L2
274
[EuL2(NO3)](NO3)2ꢂH2O 274
L3
275
[EuL3(NO3)](NO3)2ꢂH2O 276
L4
273
[EuL4(NO3)](NO3)2ꢂH2O 272
L5
279
[EuL5(NO3)](NO3)2ꢂH2O 274
L6
275
[EuL6(NO3)](NO3)2ꢂH2O 277
280
320
360
400
L7
276
wavelength/nm
[EuL7(NO3)](NO3)2ꢂH2O 277
L8
276
Fig. 2. The UV–Vis spectra of [EuL7(NO3)](NO3)2ꢂH2O (a) and L7 (b).
[EuL8(NO3)](NO3)2ꢂH2O 276
frequency by 2 cmꢁ1 and 5 cmꢁ1 after complexation. So the results
above indicate that the oxygen atoms in ArAOAC take part in coor-
dination. But the m(CASAC) absorption bands remain unchanged,
indicating the fact that the triazole and thiadiazole rings have large
sterically hindered effect, which prevents N and S atoms in the
fused rings from coordinating to Eu(III) at the same time. The
absorption bands lying at approximately 1484 cmꢁ1 and
1385 cmꢁ1 are observed in complexes, which are attributed the
1.2
0.8
0.4
0.0
b
a
asymmetric vibration absorption (mas) and symmetric vibration
absorption (
m
s). In terms of [EuL2(NO3)](NO3)2ꢂH2O and [EuL5
(NO3)](NO3)2ꢂH2O, the frequency separation
[
D
m
=
m1
ꢁ
m4]
between the asymmetric and symmetric stretching of this group
can be made to distinction between these binding states. The dif-
-0.4
280
ference between m1 and m4 of the two complexes are approximately
320
360
400
101 cmꢁ1 and 94 cmꢁ1 which can be suggested that the coordi-
nated NOꢁ3 ions in the europium complexes are bidentate coordina-
tion [17], establishing that the NOꢁ3 groups in the solid complexes
coordinate to rare earth ions as bidentate ligands [18]. The band at
470 cmꢁ1 in the two complexes can be attributed to the EuAO
stretching vibration [19].
wavelength/nm
Fig. 1. The UV–Vis spectra of [EuL2(NO3)](NO3)2ꢂH2O (a) and L2 (b).
As for the europium nitrate complexes, the bands for
a subtle shift compared to the ligands.
m(C@N) show
Additionally, the infrared spectra of the complexes exhibit a
broad band between 3400 and 3444 cmꢁ1, which indicate that lat-
tice water molecules are existent in the molecular unit of com-
plexes, in agreement with the elemental analysis discussed in the
previous section.
In the IR spectra of the complexes [EuL2(NO3)](NO3)2ꢂH2O and
[EuL5(NO3)](NO3)2ꢂH2O, the bands for
m(C@N) shifted from 1590,
1583 cmꢁ1 in the free ligand to 1611 cmꢁ1 = 21, 18 cmꢁ1) in
(Dm
the complex, respectively, thus indicates that the nitrogen atoms
in pyridine are coordinated with europium ions. The absorption
On the basis of the results above, we reached the conclusion
that the ligands coordinated to Eu(III) ions by the nitrogen atoms
of the pyridine, and thiadiazolo ring, the oxygen atoms of phenoxy
methylene and one molecule nitrate, and the coordination number
of the Eu(III) ions is 6. The elemental analysis and thermal gravi-
metric analysis data results lead us to propose the most probable
coordination structure of the europium complexes of the ligands
shown in Fig. 5.
bands for
ligands L2 and L5, respectively, shifted to 1242 cmꢁ1 and
1258 cmꢁ1 = 1, 7 cmꢁ1) in the corresponding complexes, this
m
(C@NAN), which appear at 1241, 1251 cmꢁ1 in the
(Dm
confirms that the nitrogen atoms of C@NAN are chelated with
the europium ions successfully. In addition, the absorption peaks
of
numbers, the vibration
m
(ArAOAC) shift by 14 cmꢁ1, 18 cmꢁ1 towards higher wave
m
(ArAH) are shifted towards higher

492
D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
Table 3
The IR spectral data of the free ligands L1–8 and their nitrate complexes (cmꢁ1).
ꢁ
Compounds
mC@N
mC@NAN
mArAOAC
mCASAC
mArAH
mOH
mNO
3
L1
1598
1598
1246
1247
1080
1081
690
691
3138
3175
3414
3413
1479, 1385, 1025, 817
1484, 1383, 1030, 815
1477, 1385, 1026, 835
1492, 1385, 1024, 813
1479, 1385, 1024, 830
1489, 1385, 1023, 830
1482, 1385, 1023, 823
1474, 1386, 1024, 822
[EuL1(NO3)](NO3)2ꢂH2O
L2
1590
1611
1241
1242
1083
1097
685
685
3122
3155
3413
3411
[EuL2(NO3)](NO3)2ꢂH2O
L3
1594
1614
1232
1235
1032
1058
688
688
3121
3191
3444
3408
[EuL3(NO3)](NO3)2ꢂH2O
L4
1611
1610
1259
1260
1054
1054
687
687
3119
3152
3413
3428
[EuL4(NO3)](NO3)2ꢂH2O
L5
1593
1611
1251
1258
1081
1099
687
688
3118
3165
3422
3417
[EuL5(NO3)](NO3)2ꢂH2O
L6
1595
1609
1243
1248
1040
1059
687
687
3138
3165
3414
3400
[EuL6(NO3)](NO3)2ꢂH2O
L7
1594
1611
1246
1256
1052
1059
680
681
3140
3149
3409
3411
[EuL7(NO3)](NO3)2ꢂH2O
L8
1634
1610
1244
1245
1050
1051
680
680
3152
3182
3409
3410
[EuL8(NO3)](NO3)2ꢂH2O
a
N
N
N
N
S
N
S
N
N
N
N
Eu
b
(NO 3)
2.H2O
O
O
O
O
N
O
4000 3500 3000 2500 2000 1500 1000 500
wavenumber/cm-1
Fig. 3. IR spectra of [EuL2(NO3)](NO3)2ꢂH2O (a) and L2 (b).
R
R
R=H, CH3, OCH3, NO2, F, Cl, Br, I
Fig. 5. The most probable coordination structure of the complexes.
a
only the complexes [EuL3(NO3)](NO3)2ꢂH2O and [EuL5(NO3)]
(NO3)2ꢂH2O are selected for investigation in detail as a representa-
tive example to perform thermal decomposition analysis (Figs. 6
and 7).
b
The TG–DSC curves of the title complexes show that the com-
plexes undergo multi-stage changes. The initial mass loss occurs
shown (observed value 1.71% and 1.85%) in the TG curves between
70 °C and 150 °C reveals the loss of one crystal water molecule
(calculated value 1.88% and 1.93%), it was an endothermic process
observed from the DSC curves.
4000 3500 3000 2500 2000 1500 1000 500
A further mass loss stage of [EuL3(NO3)](NO3)2ꢂH2O located at
around 304 °C and 330 °C, with mass loss percentage of 13.02%
and 6.55%, these values well agree with the loss of two external
nitrates molecule and one internal nitrate molecule, the calculated
values were 12.97% and 6.49%, similarly, the second mass loss
stage of [EuL5(NO3)](NO3)2ꢂH2O at around 302 °C and 360 °C, with
mass loss percentage of 13.58% and 6.62%, close to the theoretical
values 13.31% and 6.65%. There are couple of main successive mass
loss stages at 383 °C and 428 °C for the two complexes, respec-
tively. This stage basically agrees with the values of 41.84% and
40.34%, which is ascribed to the decomposition of free ligand. Fur-
ther heating two complexes to 800 °C, the weight ratio of residues
were 42.71% and 39.88%, the values basically agree with calculated
wavenumber/cm-1
Fig. 4. IR spectra of [EuL5(NO3)](NO3)2ꢂH2O (a) and L5 (b).
Thermogravimetric analysis
To study the thermal behavior of the title complexes, TG–DSC
measurements had been carried out for the Eu(III) complexes
within the temperature range from ambient temperature up to
800 °C under a nitrogen atmosphere. All the TG–DSC data of com-
plexes were listed in Table 4. The TG–DSC curves showed that the
samples gave similar thermal decomposition behavior; herein,

D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
493
Table 4
Differential thermal–thermal gravimetric analysis data for the complexes.
Complexes
Endothermic peak (°C)
H2O found (calc.) (%)
NOꢁ3 found (calc.) (%)
Ligand found (calc.) (%)
Residual weight (calc.) (%)
[EuL1(NO3)](NO3)2ꢂH2O
[EuL2(NO3)](NO3)2ꢂH2O
[EuL3(NO3)](NO3)2ꢂH2O
[EuL4(NO3)](NO3)2ꢂH2O
[EuL5(NO3)](NO3)2ꢂH2O
[EuL6(NO3)](NO3)2ꢂH2O
[EuL7(NO3)](NO3)2ꢂH2O
[EuL8(NO3)](NO3)2ꢂH2O
88, 266, 400
85, 265, 348
263, 330, 383
265, 295, 436
265, 360, 428
265, 395
2.06(2.01)
2.45(1.95)
1.71(1.88)
2.32(1.83)
1.85(1.93)
1.77(1.87)
1.76(1.71)
2.06(1.57)
20.50(20.76)
19.96(20.13)
19.57(19.46)
18.75(18.86)
20.02(19.96)
19.85(19.29)
17.55(17.66)
16.30(16.22)
35.89(37.94)
39.25(39.82)
37.14(41.84)
39.07(42.95)
38.35(40.34)
40.53(42.33)
45.22(47.20)
47.85(51.52)
42.87(39.29)
38.22(38.10)
42.71(36.82)
40.26(36.36)
39.88(37.77)
38.09(36.51)
36.14(33.43)
34.21(30.69)
89, 350, 450
92, 268, 296
Table 5
100
80
60
40
20
0
TG
DSC
Luminescence data for the title complexes.
4
Complexes
kex (nm) 5D0—7F1
kem (nm) I (a.u.)
5D0—7F2
kem (nm) I (a.u.)
3
2
1
0
[EuL1(NO3)](NO3)2ꢂH2O 369
[EuL2(NO3)](NO3)2ꢂH2O 369
[EuL3(NO3)](NO3)2ꢂH2O 369
[EuL4(NO3)](NO3)2ꢂH2O 369
[EuL5(NO3)](NO3)2ꢂH2O 369
[EuL6(NO3)](NO3)2ꢂH2O 369
[EuL7(NO3)](NO3)2ꢂH2O 369
[EuL8(NO3)](NO3)2ꢂH2O 369
594.5
594.5
594.5
595
391.7
118.2
2.188 620
44.11
905.5
833.1
118.7
14.26
620.5
620.5
1479
514.6
3.262
136.4
3409
3659
495.1
60.87
621
594.5
595
620.5
621
594.5
594.5
620.5
620.5
100
200
300
400
500
600
700
800
Temperature/ oC
200
150
100
50
Fig. 6. TG–DSC curves of [EuL3(NO3)](NO3)2ꢂH2O.
100
80
60
40
20
0
4
3
2
1
0
TG
DSC
0
-50
350
360
370
380
390
400
wavelength/nm
Fig. 8. Excitation spectrum of the [EuL6(NO3)](NO3)2ꢂH2O.
100
200
300
400
500
600
700
800
Temperature/ oC
4000
3000
2000
1000
0
Fig. 7. TG–DSC curves of [EuL5(NO3)](NO3)2ꢂH2O.
values of 36.82% and 37.77%, corresponding to the formation of
Eu2O3. The results demonstrate that these complexes have a satis-
factory thermal stability, and the uniform of the structure was con-
firmed further.
Luminescence spectral studies
The luminescence spectra of the ligands and their europium
complexes in the solid state were recorded at room temperature.
The excitation and emission slit widths were 5 nm and the PMT
voltage was 700 V. Under identical experimental conditions, the
luminescence characteristics data of the title complexes are listed
in Table 5. Due to the similarity of luminescence spectra of the title
complexes, herein showed the excitation and emission spectra of
[EuL6(NO3)](NO3)2ꢂH2O in Figs. 8 and 9, respectively.
450
500
550
600
650
700
wavelength/nm
Fig. 9. Emission spectrum of the [EuL6(NO3)](NO3)2ꢂH2O.
different complexes. It is observed in Fig. 9 that the complex
[EuL6(NO3)](NO3)2ꢂH2O shows the characteristic emission spectra
of Eu(III) ions, centered at about 595 nm and 620 nm, assigned to
5D0 ? 7F1 and 5D0 ? 7F2 transitions, respectively, indicating that
It can be seen from Fig. 8 that the complex exhibits a broad
band in the 270–380 nm region (kmax = 369 nm), so the peak height
at 369 nm was used to measure the luminescence intensities of the

494
D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
the ligand is a kind of good chelator which can absorb and transfer
energy to Eu(III) ions. Additionally, we could see the intensity of
the hypersensitive 5D0 ? 7F2 transition (electric dipole) is greater
than that of the 5D0 ? 7F1 transition (magnetic dipole) and the for-
mer is about 4.39 times higher than the latter, which suggests that
Eu(III) does not lie in a centro-symmetric coordination site [20].
Furthermore, the relative intensity of 5D0 ? 7F2 transition is all
higher than that of 5D0 ? 7F1 transition among these complexes
[EuL1–8(NO3)](NO3)2ꢂH2O, this confirmed that Eu(III) has the lower
symmetric coordination environment. The asymmetric microenvi-
ronment causes the polarization of the Eu(III) ions under the influ-
ence of electric field of the surrounding ligands, which increases
the probability for the electric dipole transition [21].
is the area of fluorescence integral, and A is the absorbance at the
exciting wavelength of UV–Visible absorption. The fluorescence
spectral data of the europium complexes were measured at room
temperature in DMSO solution with excitation and emission slit
widths were 5.0 nm and the calculated results of fluorescence
quantum yield (Ufx) of the complexes are listed in Table 6.
As shown in Table 6, we found that the fluorescence quantum
yields of Eu(III) complexes with electron-withdrawing groups are
larger than that of the Eu(III) complexes with electron-donating
groups, which is consistent with the fluorescence data of title euro-
pium complexes measured in the solid state and DMSO solution.
The halogen-containing complexes show higher fluorescence
quantum yield and substantially increased with the decrease of
atomic number in substituents. Meanwhile, the fluorescence quan-
tum yield of [EuL6(NO3)](NO3)2ꢂH2O is the highest 0.522 among the
complexes [EuL5–8(NO3)](NO3)2ꢂH2O, which is due to the fact that
In view of the theory of antenna effect [22], the intensity of the
intra-molecular energy transfer between the triplet levels of the
ligand and the emitting level of europium ions is one of the key fac-
tors influencing the luminescence properties of lanthanide com-
plexes. The ligands possessed multiple aromatic rings, triazole
and thiadiazole rings with a rigid planar skeleton structure that
has a strong antenna effect. In order to obtain luminescence, the
lowest triplet state energy of the ligand, must be close or lie above
the resonance energy of the Eu(III) ions [23]. Among the eight com-
plexes, it is clearly observed that the complexes substituted by the
electron-withdrawing group have stronger luminescence intensi-
ties, this may be ascribed to triplet level of the ligands be lowered
by the electron-withdrawing groups, thus the triplet state energy
level of the ligands approaching to the resonance level of Eu(III),
which makes the energy transition from ligands to Eu(III) more
easily. While the luminescence intensity of the nitro-substituted
europium complex became weak, which may be affected by the
strong inductive effect of nitro, thus the triplet energy level of
L6 possesses the stronger p–
p conjugation effect compared with
ligand L5 and the heavy atom effect in the ligand L6–8, and the trip-
let energy level of chlorine-substituted ligand matches better to
excite the state level of Eu(III), thus the corresponding complex
shows good light-emitting ability.
Electrochemistry properties studies
Cyclic voltammetry (CV) experiments were conducted to detect
the electrochemical properties of the europium complexes. The
oxidation potential EOX of the europium complexes were measured
by cyclic voltammograms, shown in Fig. 10 and the electrochemi-
cal data of the complexes is summarized in Table 7.
From Table 7, it can be seen that the HOMO energy level and
oxidation potential of [EuL2–3(NO3)](NO3)2ꢂH2O are higher than
the values of [EuL1(NO3)](NO3)2ꢂH2O, on the contrary, the HOMO
energy level and oxidation potential of [EuL4–8(NO3)](NO3)2ꢂH2O
ligand was significantly reduced, because such small
Dm
(T1–5D0)
could result in the non-radioactive deactivation of the europium
emitting state via a back-energy transfer process (T1 5D0) and
quench the luminescence of the europium nitrate complex [24].
The luminescence intensities of the europium complexes with bro-
mine and iodine substituted group became weaker; this may be
due to heavy atom effect. The halogen atoms of the relatively larger
atomic mass could enhance the spin-orbit coupling in the mole-
Table 6
Fluorescence quantum yields of Eu(III) complexes in DMSO solution.
Compounds
Excitation
wavelength k
(nm)
Fluorescence
intensity I
(a.u.)
Fluorescence
quantum yield
U
cule, which leads to the decrease of
Dm
(T1–5D0) and the lumines-
[EuL1(NO3)](NO3)2ꢂH2O 319
[EuL2(NO3)](NO3)2ꢂH2O 320
[EuL3(NO3)](NO3)2ꢂH2O 324
[EuL4(NO3)](NO3)2ꢂH2O 328
[EuL5(NO3)](NO3)2ꢂH2O 329
[EuL6(NO3)](NO3)2ꢂH2O 318
[EuL7(NO3)](NO3)2ꢂH2O 321
[EuL8(NO3)](NO3)2ꢂH2O 319
1454
1203
914
1096
1677
1712
1586
1104
0.392
0.314
0.287
0.303
0.475
0.522
0.410
0.308
cence intensities. The luminescence intensities of the complexes
with electron-donating groups was decreased probably result from
increasing of the triplet energy level when introduced electron-
donating groups, so the triplet energy level far from the lowest res-
onance level of Eu(III). Considering the above results, it is con-
cluded that the luminescence intensity of europium complexes
with chlorine substituent group is the strongest in the all eight
complexes, which indicate that the triplet energy level of chlo-
rine-substituted ligand matches better to the lowest resonance
level of Eu(III).
a
8
4
b
c
Fluorescence quantum yield studies
The fluorescence quantum yields of the title europium com-
plexes were determined by the reference method using quinine
sulfate (dissolved in 0.5 M sulfuric acid at a concentration of
10ꢁ6 M, assuming Ufstd = 0.55) as a standard. The quantum yields
of the Eu(III) complexes were calculated by the following equation
[25]:
d
e
f
0
g
h
-4
-8
n2x Fx Astd
Ufx
¼
ꢂ
ꢂ
ꢂ
Ufstd
n2std
Fstd Ax
-2.4
-1.6
-0.8
0.0
0.8
1.6
In the formula, Ufx is the fluorescence quantum yield of sample,
subscripts std and x refer to the standard and the unknown, respec-
tively, n represents the refractive indices (nstd = 1.337, nx = 1.48), F
E/V
Fig. 10. Typical cyclic voltammogram of [EuL1–8(NO3)](NO3)2ꢂH2O (a–h).

D. Yan et al. / Journal of Molecular Structure 1074 (2014) 487–495
495
Table 7
group is bound in a bidentate manner to the central Eu(III) ions
in the coordination sphere while another two nitrate are in the
outer coordination sphere in the complexes. The luminescence
data reveals that the title complex with chlorine-substituted group
possesses the strongest luminescence intensity and the highest
fluorescence quantum yield (0.522). Additionally, the test results
of cyclic voltammograms of the title complexes show that the
introduction of electron-donating groups can increase the HOMO
energy levels, LUMO energy levels and the oxidation potential of
the complexes, however, the result of introduction of electron-
withdrawing groups was just opposite. These results demonstrate
that the title complexes possess good application prospects and
theoretical research value.
Electrochemical properties of the title complexes.
Complexes
konset
(nm)
EOX
EHOMO
Eg
ELUMO
(ev)
(v)a
(ev)b
(ev)c
[EuL1(NO3)](NO3)2ꢂH2O 251
[EuL2(NO3)](NO3)2ꢂH2O 253
[EuL3(NO3)](NO3)2ꢂH2O 251
[EuL4(NO3)](NO3)2ꢂH2O 253
[EuL5(NO3)](NO3)2ꢂH2O 251
[EuL6(NO3)](NO3)2ꢂH2O 254
[EuL7(NO3)](NO3)2ꢂH2O 251
[EuL8(NO3)](NO3)2ꢂH2O 250
0.750
0.762
0.754
0.703
0.718
0.725
0.739
0.745
5.490
5.502
5.494
5.443
5.458
5.465
5.479
5.485
4.940
4.901
4.940
4.901
4.940
4.882
4.940
4.960
0.550
0.601
0.554
0.542
0.518
0.583
0.539
0.525
a
b
c
Calculated using equation EHOMO = 4.74 + eEOX
Calculated using equation ELUMO = EHOMO ꢁ Eg.
Calculated using equation Eg = 1240/konset (eV). (The konset is the maximum
.
peak start value of ultraviolet absorption spectrum).
Acknowledgements
The authors are grateful for the financial support of the National
Natural Science Foundation of China (Nos. J1103312; J1210040;
and 21341010), the Innovative Research Team in University (No.
IRT1238), the Science and Technology Project of Hunan provincial
Science and Technology Department (No. 2012GK3156) and chem-
ical excellent engineer training program of Hunan University. We
also thank Dr. William Hickey, the U.S. professor of HRM, for the
English editing on this paper.
are slightly declined. The results may be due to the various substi-
tuent types of the ligands, and the electron-donating group is ben-
efit to improving the oxidation potential, and the introduction of
electron-withdrawing group will reduce oxidation potential, in
other words, the characters of the substituents affect the HOMO
level of the europium complexes. In the series of title europium
complexes, methyl-substituted europium complex shows the
highest HOMO energy level and oxidation potential, nitro-substi-
tuted europium complex exhibits the lowest HOMO energy level
and oxidation potential. The HOMO energy level and oxidation
potential of halogen atom-substituted europium complexes was
determined in the following order: F < Cl < Br < I. The LUMO repre-
sents the ability to donate an electron. The smaller the LUMO and
HOMO energy gap computed in the present investigation suggest
that the HOMO electrons can be easily excited. The energy gap of
the europium complexes range from 4.882 eV to 4.960 eV, which
indicated that the electron-donating groups and electron-with-
drawing groups have no apparent effect on energy gap. Moreover,
the introduction of electron-donating groups can increase the
HOMO energy levels, LUMO energy levels and the oxidation poten-
tial of the complexes. The introduction of electron-withdrawing
groups tend to decrease HOMO energy levels, LUMO energy levels
and the oxidation potential of the complexes. The LUMO energy
energy level of the europium complexes with chlorine-substituted
group is the highest among halogen atom-substituted europium
complexes.
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Conclusion
Eight novel 2,6-bis-(4-(4-substituted phenoxymethyl)-1,2,4-
triazolo[3,4-b][1,3,4]thiadiazole)-pyridine derivatives ligands were
designed and synthesized, and characterized by means of 1H NMR,
mass spectra, elemental analysis, UV spectra and infrared spec-
tra(IR). The title europium(III) complexes [EuL1ꢁ8(NO3)](NO3)2ꢂH2O
were synthesized, and then characterized based on elemental anal-
ysis, EDTA titrimetric analysis, UV spectra, infrared spectra, ther-
mal gravimetric analysis and molar conductivity. According to
the IR spectra data, the coordination of the ligands to Eu(III) ions
was occurring at one nitrogen atom of the pyridine ring, two nitro-
gen atoms of thiadiazole rings and an oxygen atom of phenoxym-
ethylene. Thermal study indicated that complexes are thermally
stable. Spectral study and thermal data showed that one nitrate